Implement WP authenticator

[ssnepenthe]

I envision it working like this:

Dev configures user cap required to view metadata. Probably 'manages_options' by default, or maybe we add a clockwork-specific capability.

Username and password would both be required. If credentials match, we fetch or create a clockwork token from user meta. This is the token returned by Auth::attempt().

The Auth::check() method would do a user lookup by clockwork token meta. User caps should probably be verified here as well in case the user role has recently changed.